1. Field of the Invention
The subject invention relates to a spark plug for a spark-ignited internal combustion engine, and more particularly toward a spark plug having a fired-in suppressor seal contained in the insulator between a lower center electrode and an intermediate connecting pin.
2. Related Art
A spark plug is a device that extends into the combustion chamber of an internal combustion engine and produces a spark to ignite a mixture of air and fuel. In operation, charges of up to about 40,000 volts are applied through the spark plug center electrode, thereby causing a spark to jump the gap between the center electrode and an opposing ground electrode.
Electromagnetic interference (EMI), also known as radio frequency interference (RFI), is generated at the time of the electrical discharge across the spark gap. This is caused by the very short period of high frequency, high current oscillations at the initial break down of the gap and at points of refirings. This EMI (or RFI) can interfere with entertainment radio, two-way radio, television, digital data transmissions or any type of electronic communication. In a radio for example, the EMI or RFI is usually noticed as a “popping” noise in the audio that occurs each time the spark plug fires. Ignition EMI is a nuisance and in extreme cases can produce performance and safety related malfunctions.
Levels of EMI emitted by a spark ignition system engine can be controlled or suppressed in many ways. Commonly, EMI suppression of the ignition system itself is accomplished by various methods, including the use of resistive spark plugs, resistive ignition leads, and inductive components in a secondary high voltage ignition circuit. A common type of resistor/suppressor spark plug used for the suppression of EMI contains an internal resistor element placed within the ceramic insulator between the upper terminal stud and the lower center electrode.
While internal resistor/suppressor spark plug designs are well known, practical considerations have frustrated the ability to integrate a resistor in small diameter spark plugs, for example those sized to fit a 12 mm or less (10 mm, 8 mm, etc.) diameter threaded bore. In particular, the fairly large cross-sectional area required for the resistor inside the insulator weakens the structural integrity of the ceramic insulation by creating a thin wall section precisely in the region of an insulator which is often highly stressed during assembly and operation. Also, by creating such a thin wall, the amount of voltage the part can sustain is reduced. Furthermore, reducing the cross-sectional area of the resistor demands a corresponding reduction in the diameter of the upper terminal stud which is used for the cold pressing and tamping operations. Thus, during the cold pressing operation where loose, granular resistor material is compressed by the upper terminal stud, and then later hot pressed to produce the so-called “fired in suppressor seal” buckling of a reduced diameter upper terminal stud is possible, both during initial insertion into the unfired powder, and during the hot pressing operation.
Additionally, the relatively long, unitary upper terminal studs are typically heated along with the sealing glasses in a furnace prior to the hot pressing operation. Heating of the upper terminal stud results in oxidation and discoloration of the terminal. In addition to detracting from the aesthetic appearance of the exposed terminal stud, the oxidized terminal (post heating) presents a rougher surface finish that requires more force to connect a spark plug wire lead.
FIG. 1 represents an example of a prior art spark plug construction taken from the Applicant's own U.S. Publication No. 2005/0093414, published May 5, 2005, the entire disclosure of which is hereby incorporated by reference. This publication illustrates use of an intermediate connecting pin lodged in the central passage of the insulator, generally midway between a center electrode at the lower end of the insulator and a terminal post at the upper end of the insulator. The contact pin fits snugly within the central passage and includes a threaded lower portion, which is embedded in the conductive glass seal above the center electrode. As described in Paragraph [0021] of that publication, the glass seal may have several distinctive layers to provide desirable electrical characteristics such as suppression of high frequency interference. While this design represents a marked improvement over then-existing prior art instructions, there remain certain shortcomings. For example, the smooth piston-like fit of the connecting pin within the central passage has the potential to trap gasses during assembly, thereby creating gas bubble inclusions within the glass seal which degrade electrical performance during use. This may also cause stress that could burst the side walls of the ceramic insulator under pressure. Furthermore, in high thermal cycling events over prolonged use, it is possible that the connection between the threaded lower portion of the connecting pin and the enveloping glass seal may break loose due to differing rates of thermal expansion and the thermal stresses that result during cycling.
FIG. 2 represents another prior art design such as that depicted in U.S. Pat. No. 3,915,721, issued Oct. 28, 1975. In this example, a connecting pin having a lateral dimension substantially smaller than the internal diameter of the central passage is provided. Due to the sizeable clearance space between the connecting pin and the side walls of the central passage, there is no chance for gasses to become trapped during the assembly process. However, a design of the type depicted in FIG. 2 presents certain difficulties of its own. For one example, the clearance space affords an opportunity for the connecting pin to tip or become uncentered during assembly, as shown by broken lines in FIG. 2. Loss of control of the position of the pin during processing can result in unacceptable variations in the resistance of the finished spark plug.
Another shortcoming exhibited by both prior art designs depicted in FIGS. 1 and 2 relates to the unique challenges confronted when attempting to downscale the size of the spark plug. These issues are mentioned above and include a thinning of the insulator wall in critical areas, such that the dielectric capacity of the insulator material may be breached. For illustrative purposes, a dielectric puncture zone is depicted in FIG. 2. Furthermore, these thinned sections of insulator wall become failure points when, during assembly, the shell is clamped about the exterior of the insulator, thereby placing a region of the insulator in compression. Thin sections of the insulator wall are thus susceptible to catastrophic failure during compression loading. The prior art designs depicted in FIGS. 1 and 2 are fairly typical, and illustrate the central passage as having a continuous interior diameter from the upper terminal end of the insulator to the head of the center electrode. Thus, as the spark plug is scaled down to accommodate smaller sized applications, the proportional decrease in wall thickness of the insulator can result in dielectric breach and/or compression load failure.
Accordingly, the current trend toward reduced diameter spark plugs introduces many practical difficulties. The insulator wall thickness area of fired-in suppressor seal components experiences weakened structural integrity. Furthermore, the current technique of heating the upper terminal stud together with the sealing glasses in a furnace results in oxidation and discoloration of the upper terminal stud which detracts aesthetically and contributes to connector installation problems. Therefore, there is a need in this field to provide a spark plug assembly that can implement the well known fired-in suppressor seal features in a small diameter package, and which further avoids problems associated with heating the upper terminal stud in a furnace.